Where to farm experience in Pokemon X/Y?

Solution 1:

You have a few options for Exp farming in XY. First however, let's look at what you will need to farm Exp effectively:

Setup

Items

Exp. Share - rewards Pokémon that did not take part in the battle with half the Exp.
- Received as part of the story after the first Gym, you can't miss it. Just make sure it's turned on.
Lucky Egg(s) - A hold item that increases Exp gains for a single Pokémon.
- There is one in Coumarine City, get it from a girl in the Hotel.
- More can be farmed from Chanseys/Happinys using Thief/Covet in Normal Friend Safaris at a 5% chance of holding them (20% if your lead Pokémon has Compound Eyes)
- You can also trade for them. Players with ORAS can easily farm them from Pellipers using DexNav
Amulet Coin or Luck Incense
- Doubles Prize Money received after battle if the holder takes part in the battle
- Whilst not needed for Exp farming per se, you do want to be making a profit (or just breaking even) on restaurant battles and/or the Battle Chateau due to the high entry cost
- Luck Incense can be bought from a booth on the pier in Coumarine City. Amulet Coin is found on the second floor in Parfum Palace (check the fireplace inside the first room on the far left upstairs)
- It doesn't matter which you have, they have the same benefit and do not stack with one another

O-Powers

Find and talk to Mr Bonding in the locations below to receive the O-Power. He has brown hair and is wearing a Pink Suit.

Exp O-Power - Anistar City Pokémon Center
Prize Money O-Power - Cyllage City Hotel. Again, not technically a requirement for Exp farming, but some of the better areas have a high fee for entry (Restaurants and Challenge Writs), and it's useful to have around just in case.

Pokémon

Ideally you want 1-2 Pokemon who can knock out opposing Pokemon quickly, with the rest of your party consisting of Pokemon you wish to Exp train.

Members from your original in-game team would be a good starting point.
As you train better Pokemon, you can build more specialised Pokemon, with movesets and abilities specifically for taking on the Restaurant/Chateau.
Pick Pokemon that complement each other's typings in case you come across something that your primary attack of choice doesn't hit well.
Spread moves such as Earthquake have their power drastically dropped in double and triple battles. Something to watch for in the restaurant battles. You're better off picking off the opposing team one-by-one with single-hit moves
Try not to use moves that lower your primary attack stat, such as Draco Meteor or Overheat until later in the battle (if at all), you'll need that stat to finish off the rest of the Pokemon

How to Farm Exp

Choose your primary and secondary 'attacking' Pokemon
Give one of them Amulet Coin or Luck Incense
Fill your other party slots with the Pokemon that you wish to level up quickly
Give each Pokemon a Lucky Egg
Enter your farming area
Save
- 6.1 Save again because you aren't sure whether you saved or not
Activate O-Power(s)
Battle
Repeat from Step 6 until satisfied.

Note that you can also boost a Pokemon's Affection level through Pokemon Amie, which will also add a 20% boost to Experience when it is maxed out, however I don't recommend this: the time taken could instead be spent actually earning the Exp.

Where to Farm Exp

Restaurant Le Wow

Le Wow is located on Hibernal Avenue in the North-East part of Lumiose city. This is probably the best place to farm in terms of return on investment.

Initial Cost: $100,000 ($90,000 if previously completed with 'perfect' timing)
Pokémon Levels: 63
Experience Gains, Comparing Pokémon that participated in-battle vs only Exp. Share (assuming level 3 Exp. O-Power):
- Without Lucky Egg: 70,626 / 35,302
- With Lucky Egg: 105,928 / 57,826
- Traded Pokémon with Lucky Egg: 158,888 / 79,412
Money Awarded: ~$170,000 after selling Mushrooms

Make sure you are doing Triple or Rotation battles - With Doubles you will only fight 2/3 of the trainers' Pokémon, missing out on 5 knockouts, including an Audino, which gives lots of Exp!

Battle Chateau

Initial Cost: Nothing, with costs for more challengers/tougher challengers
Pokemon Levels: 15-60 (80 with Black Writ of Challenge)

The Chateau on Route 7 can be a good source of Exp and income, if you level it up high enough.

Battle Trainers to increase your rank.
At high enough ranks (Marquis/Marchioness and up), and you will start facing Gym leaders, Elite Four, and Champions.
The ranks are:
- Baron/Baronness (Beginner)
- Viscount/Viscountess (Defeat 5 trainers)
- Earl/Countess (Defeat 20 trainers)
- Marquis/Marchioness (Defeat 40 trainers)
- Duke/Duchess (Defeat 140 trainers)
- Grand Duke/Grand Duchess (Defeat all the trainers in the Chateau)
Issue 'writs' to increase the number of trainers that appear, or increase their difficulty:
- Invitation Writs (Writ, Silver, Gold) will bring more trainers to the Chateau. The more expensive the Writ, the more trainers that will appear.
- Challenge Writs (Writ, Red and Black) will increase the level of the trainer's Pokémon by 5, 10 and 20 levels respectively. The exception to this is the Blue Writ, which will lower the trainer's Pokémon's levels by 5.

At Grand Duke level, Pokémon are level 60 normally, 65 with a Writ, 70 at Red Writ and 80 at Black Writ of Challenge. This is the highest level NPCs can reach in the game and is a fairly good source of both Exp and money once you reach higher ranks.

Elite Four

Subsequent playthroughs of the Elite Four are time consuming, but rewarding. There are 22 High-level Pokemon to knock out.

Pokémon Levels: 63-68

The Elite Four are good because they can be challenged at any time, and aren't time-based like the Chateau. However the best part about this option is the cost: Nothing. The Elite Four doesn't charge a fee so you can battle them at any point, especially if you're running low on money.

Conclusion

Resturant Le Wow is the best option, due to the ease of use and the fact that the O-Power doesn't expire between battles, which is the biggest problem with the Battle Chateau and the Elite 4: you'll be spending time waiting for O-Powers to recharge, or fighting them without the boost it gives.

The other problem with the Chateau and E4 is time, though for different reasons:

For the Chateau, It will take a while to get to Duke/Duchess levels where you will be fighting the high-level trainers you want to fight.
Whereas the Elite 4 is filled with cutscenes, dialogue and has a lengthy credits sequence, making each run-through a very long process.

PS: Note on Sushi High Roller
Sushi High Roller isn't worth it. There are only two levels difference in Pokemon compared to Restaurant Le Wow (65 compared to 63), they have the same amount of trainers & the same amount of Pokemon each, and it is super expensive to dine there, costing $500,000 up front, most of which you won't make back unless you use every single method of increasing prize money in the game:

The Lv. 3 Prize Money O-Power
Have a Pokémon use Happy Hour,
Have a Pokémon holding an Amulet Coin/Luck Incense,
Earning at least 16 of the 25 maximum rewarded Big Nuggets.